The present invention relates to a method of joining and reinforcing molded frames. More particularly, the present invention relates to a method of joining molded halves of a bicycle frame without requiring alignment elements on the frame halves, yet providing a sturdy connection between the frame halves. The present invention further relates to a method of reinforcing high stress areas in a bicycle frame formed from molded frame halves.
Bicycle frames generally are formed from substantially hollow tubular frame elements connected together in a manner depending on the material and manner of formation of the tubular elements. When the tubular elements are formed from a lightweight metal, such as aluminum, the metal is generally cast into a single-piece tubular element. Several such tubular elements are connected together, such as by welding or gluing, to form the frame of the bicycle. When the tubular elements are formed from a fiber-reinforced plastic or composite material, the elements typically are adhesively bonded into lugs. Alternatively, composite lugs may be formed over the pre-fabricated composite tubular elements. Because all of the tubular elements must be joined together to form the finished bicycle, formation of a bicycle frame from tubular elements is rather labor intensive. Additionally, the lugs or joints with which the tubular elements are joined must be relatively thick and heavy in order to provide a secure connection between the elements.
A method that eliminates the use of multiple parts that must be joined to form a finished bicycle frame involves the formation of an inflation-cured, one-piece hollow shell. This method uses composite materials typically composed of structural fibers (such as carbon fiber and fiberglass) and thermoset resins (such as epoxy) which are placed around an inflatable bladder (formed from materials such as a nylon film) or an expandable material (such as a heat-expandable foam). The assembly is placed in a cavity mold and either the bladder is inflated or the heat applied to cure the thermoset resin causes the expandable material to expand. The fiber and resin are thus pressed by the expanding base structure (the bladder or expandable material) against the cavity mold walls and thereby conformed to the shape of the mold during the curing of the resin. Once curing is complete, the mold is opened and the frame is removed, with the bladder or expandable material permanently remaining on the interior walls of the frame. Because the fibers are only strong in the fiber direction (either one or two directions, depending on the fiber tape or cloth available), several fiber layers, each having a different directional orientation, must be used in order to result in the desired frame structural strength. The resulting wall thickness is relatively thick, particularly in bent or angled areas (generally at the areas where joints are used in a frame formed from several tubular elements), and the frame is thus heavy, even though relatively heavy lugs or joints are not used. Because the frame remains in the mold for several hours (making the mold unavailable for use in forming another frame), and several layers must be applied, production costs typically are more expensive than for joined-tubing frames. Moreover, the frames are typically brittle and subject to cracking from use and may shatter on impact.
A bicycle frame that avoids many of the above-described disadvantages of prior art frames is formed from two molded partial shells that are joined to form a hollow finished frame. Such molded elements are typically formed from a material, such as plastic or carbon fiber, that is molded into open-sided halves because of the molding process. Each half has a plurality of interconnected half-tubes, each half-tube having a pair of substantially parallel longitudinal free edges extending along the longitudinal axis of the respective half-tube. The two half-frames are connected together, such as by gluing, along the free edges to form the completed, formed frame of the bicycle.
The longitudinal free edges of bicycle half-frames are typically provided with alignment elements that facilitate alignment of the half-frames so that the exterior of the formed frame is substantially smooth. Such alignment elements may include pins in one half and corresponding receiving holes in the other half, such as shown in U.S. Pat. No. 5,464,240 to Robinson et al. These alignment elements are rather small because of the thinness of the walls of the tubular elements, and thus are relatively difficult to manufacture without defects. Occasionally there are manufacturing defects that result in too many pins or not enough holes. In that case, the additional pins must be cut off, thus requiring yet another assembly step. Moreover, alignment of the frame halves requires the additional step of aligning the relatively small pins with their corresponding holes. Such alignment is time consuming and therefore labor intensive. Another disadvantage of using small pins is that they may break before or during alignment.
Typically, half-frames of a bicycle are glued together at their adjoining longitudinal free edges to form a glued butt joint. When the halves are joined and pressed together, the glue spreads along the edges, as usual. The glue often extends past the abutting edges to the exterior of the tubes (the rounded exterior of the frame visible to the consumer/user). In order to create a neater appearance, the exterior of the now joined frame halves must be cleaned or smoothed to improve the appearance of the frame. The frame can then be painted in the usual manner.
Bicycles frames that are formed from molded half-frames also typically include internal reinforcements, such as the protrusions or webs and corresponding slots shown in the above-mentioned Robinson Patent. As with the pins and holes, the internal reinforcements also make manufacture of the halves, and their connection to each other, more complicated. Moreover, although such reinforcement elements add to the strength of the frame, they also add to the weight of the frame, thereby making the bicycle heavier and thus more difficult to ride.
Another reinforcement element that may be provided is a reinforcement "box." Reinforcement boxes may be formed as substantially tubular elements that extend from the interior surface of a frame half for connection with a corresponding box in a corresponding frame half. Preferably, the box has a longitudinal axis that is substantially perpendicular to the longitudinal axis of the half-tubular element of the frame half from which the box extends, and thus the connected corresponding boxes form a reinforcing element that is substantially perpendicular to the plane of the completed frame. A reinforcement box may be provided at any desired location. Typically, a box is at least provided in the chain stay area and in the seat stay area adjacent the adjoining tubes (i.e., the bottom bracket shell and seat tube, respectively). Provision of such a box strengthens the tubular frame. However, the chain stay and seat stay areas are generally subjected to substantial stresses, and thus are prone to having cracking problems despite the provision of boxes. Reinforcements in addition to the boxes are thus recommended. However, the additional reinforcements used in known bicycle frames add to manufacturing costs and the weight of the frame.